We have studied the electric field-controlled optical nonlinearity in a homogeneously aligned ferroelectric Liquid crystal (FLC), SCE 13, for understanding the second harmonic generation (SHG) process at a molecular level. From the SHG intensity profiles, measured as a function of the azimuthal angle for rotation at an oblique incidence, all the relevant nonlinear optical (NLO) coefficients are determined self-consistently. It is found that the field dependence of the NLO coefficients can be described in terms of the third-order nonlinearity which couples linearly with a static electric field. The magnitude of the third-order NLO coefficient is of the order of 10(4) pm(2)/V-2, which is consistent with the measured value for a racemic version SCE 13R.